What is Metal Extrusion?
Definition: Metal extrusion is a bulk deformation operation in which a block of metal is forced to flow through a die opening to produce a desired cross sectional shape under high pressure.
In metal extrusion, the metal is squeezed through an orifice in a die, by applying a pressure with the help of a punch.
By using metal extrusion technique, brittle metals can also be extruded and the metals with complex shapes can be created with good surface finish.
Introduction to Extrusion
The commercial use of extrusion was first started in the early 19th century with the extrusion of lead pipes.
After the year 1930, the extrusion of steel was possible when special extrusion chambers were designed to resist high temperature and pressure.
In extrusion operation, the heated metal is forced or squeezed out through a die.
By extrusion technique, components like tubes, rods, or other desired shapes can be manufactured.
The opening of the die is referred to as a cross section of the required product, and this extrusion of material is similar to pressing a toothpaste out of the tube.
Extrusion principle can be used in various fields as mentioned below.
- Metals: Extrusion of metals is used for making automobile parts, metal pipe manufacturing, etc.
- Plastics: Plastic extrusion is used for manufacturing of plastic pipes, frames, etc.
- Foods: In foods, the extrusion principle is used in preparing pastas, noodles, jelly, etc.
Examples of Extrusion
#1) Aluminum frames
#2) Whiteboard frames
#3) Door and window frame
#4) Metal section bars
#5) Hollow cross-sections
#6) Hollow pipes
#7) Gear blanks
Types of Metal Extrusion
7 types of extrusions are listed below.
- Direct or forward extrusion
- Indirect or backward extrusion
- Hydrostatic extrusion
- Impact extrusion
- Lateral extrusion
- Hot extrusion
- Cold extrusion
#1) Direct or forward extrusion
Direct extrusion (also known as forward extrusion) is an extrusion operation in which the movement of ram and the flow of material are in the same direction.
First of all the hot metal is introduced in the container.
When this metal is forced with the help of ram, it gets extruded out of the die and takes the shape of the die opening.
You can see the dummy block which is attached with the ram. This dummy block prevents the contact of the hot metal with the ram.
During the direct extrusion operation, the wall friction between the heated metal and the cylinder wall is higher than the indirect extrusion.
To overcome this friction, proper lubricant is used.
If the extrusion operation is carried at very high temperature, then it is difficult to find a proper lubricant.
In such cases, molten glass is used as a lubricant.
If the direct extrusion operation is carried out at a lower temperature, then a mixture of oil and graphite is used as a lubricant.
After the extrusion operation is completed, a small amount of material is left in the chamber. This cannot be extruded and it is called butt-end scrape. It is removed and thrown away.
#2) Indirect or backward extrusion
Indirect extrusion (also known as backward extrusion) is an extrusion operation in which the movement of ram and the flow of material are in opposite directions.
In this operation, the ram is hollow and it carries a die, and the other end of the container is closed with a plate.
During the backward extrusion operation, the ram is kept stationary and the container having metal billet is moved which forces the metal to come out through the die opening.
The ram used for indirect extrusion operation is hollow which allows the metal to pass through it.
The friction forces produced in the indirect extrusion are less because the billet does not move relative to the container.
Thus the amount of forces required in indirect extrusion are less than that required in direct extrusion.
#3) Hydrostatic extrusion
In Hydrostatic extrusion, the metal billet is completely surrounded by the pressurized liquid, except where the billet contacts the die.
This metal billet is compressed by the pressurized liquid from all directions as shown in the diagram.
The hydrostatic fluids used in this process are mineral oil, ethyl glycol, castor oil mixed with alcohol, glycerine, etc.
By using hydrostatic extrusion, the brittle metals like cast iron can also be extruded easily.
The advantage of using this method is that there is no friction between the metal billet and the container surface.
For extruding a metal by this method, the billet must be prepared by tapering one end to match the size at die outlet.
#4) Impact extrusion
Impact extrusion is a type of indirect extrusion operation which is used to form hollow shapes with short lengths.
The diameter of the die is same as the outside diameter of the tube and the dimensions of the ram is same as the inside diameter of the tube.
The ram strikes the billet at a high velocity which extrudes the metal through the narrow gap between the ram and die.
Impact extrusion is suitable for soft metals like aluminum, copper, tin and lead.
#5) Lateral extrusion
In lateral extrusion, the container is in vertical position and the die is located on the side.
This process is suitable for low melting point metals like aluminum, copper, lead, etc.
#6) Hot extrusion
Hot extrusion is an extrusion operation in which the metals are heated at a very high temperature to make the extrusion process easy.
High pressure ranging between 30 to 700 MPa is maintained during the hot extrusion operation.
The high pressure and temperature creates a negative effect on the die, which decreases the effectiveness and life of the die.
Glass powder lubrication is generally used for hot extrusion operation.
Extrusion of steel, copper, aluminum, nickel, etc are done directly at a high temperature.
#7) Cold extrusion
Cold extrusion is exactly opposite to the hot extrusion.
Cold extrusion is carried out at a room temperature and it has the advantage that it does not show oxidation of the metals because of lower temperatures.
Few other advantages of cold extrusion included, better surface finish, higher strength, good dimensional accuracy, etc.
Soft materials like aluminum, copper, lead, tin can be extruded using cold extrusion method.
Metal extrusion: Image by Vivekverma239, CC0, via Wikimedia Commons